1. Field of the Invention
The present invention relates to a multi-walled steel pipe designed for use, generally, in an automotive vehicle, electric refrigerator or other machines and apparatus to feed oil and gas, as well as to a method of making a multi-walled steel pipe and a metal strip for use in making a multi-walled steel pipe. Such a multi-walled steel pipe has a small diameter, for example, of less than 15 m/m and requires sufficient pressure resistance and mechanical strength.
2. Description of the Prior Art
FIG. 4 shows an exemplary prior art steel pipe. Surfaces 11' of a metal strip are plated with copper to provide smooth surfaces 13 with a brazing layer of a few microns thereon. The metal strip is then rolled in its width direction to provide a multi-walled steel pipe. The smooth surfaces 13 or overlapping surfaces 12 are braze welded together as at 14.
However, the overlapping peripheral surfaces 12 are flat-rolled smooth surfaces 13,, and no means is provided to hold the surfaces after the metal strip has been rolled in its width direction. Such a conventional pipe is thus subject to spring back. In addition, when the pipe is heated to a melting temperature of the brazing filler metal in a furnace so as to effect braze welding 14, the filler metal is not equally distributed between the overlapping surfaces 12 of the small diameter pipe. This results in undesirable development of defects or pinholes and deteriorates pressure resistance and mechanical strength of the pipe. The prior art pipe is, thus, subject to cracking and breakage. An increased yield of pipes are also not expected with a conventional production method.